After frankly admitting that one of those mistakes was Britain's failure to deploy sufficient forces in Iraq, Dannatt told a conference at the Royal United Services Institute, Rusi, that success in Afghanistan was "not discretionary". He added: "And we must do whatever is necessary to deliver that success."

Gordon Brown has insisted more than once that Afghanistan is Britain's frontline against terrorism, our military presence there a bulwark against al-Qaida and its supporters, which is why young British soldiers are making the ultimate sacrifice there.

Brown hails the efforts of British squaddies fighting and dying in a cause he says is vital to our national security. Mysteriously, given his claims about the importance of Afghanistan, he then rejects a plan drawn up by his most senior military advisers to send 2,000 extra troops to join the 8,200 based there.

Pressure from the generals for more troops to be deployed there may seem strange. Have they not been concerned about the pressure their soldiers are already facing and the effect on their morale and training? Have they not been insisting there can be no military solution to the counter-insurgency against the Taliban?

Yes, but they also believe that to seize ground from Taliban control and provide security for development projects to take off – "clear, hold, build", as UK commanders on the ground now call it – more troops are desperately needed. "It is no secret there are not enough forces across the south [of Afghanistan]," Lieutenant General Peter Wall, deputy chief of defence staff responsible for operations, recently told an earlier Rusi event.

It is possible defence chiefs have been pushing for more troops to be deployed in Afghanistan partly out of concern for the reputation of Britain's armed forces, seriously dented among the US military after Iraq. "Credibility," Dannatt pointedly remarked (pdf) in a speech at Chatham House last month, was "linked to the vital currency of reputation. And in this respect there is recognition that our national and military reputation and credibility, unfairly or not, have been called into question at several levels in the eyes of our most important ally as a result of some aspects of the Iraq campaign." He added: "Taking steps to restore this credibility will be pivotal – and Afghanistan provides an opportunity."

Concern among Britain's defence chiefs comes as the US, as part of Barack Obama's surge, is deploying 10,000 troops to Helmand province, outnumbering the UK's military presence in an area for which Britain has been responsible over the past three years.

Dannatt warned in his Chatham House speech that the government "must not squander our increasingly scarce resources" on irrelevant military projects. He did not spell them out, though there are many in the army who question the decision to replace the ­Trident nuclear missile system.

Money, Whitehall officials say, is the reason for Brown's refusal to send more troops to Afghanistan despite his rhe­toric about the importance of that country. The suspicion is that there is more to it, that Brown is worried about the political impact against the background of the perceived unpopularity of Britain's military presence in Afghanistan.

British soldiers and humanitarian and aid agencies tell you privately that most Afghans want help for small development projects protected by their own local forces. They don't like the Taliban, but they have had little reason so far to put their trust in Nato-led foreign forces to solve their problems. According to aid agencies, just 10% of aid handed to the Afghanistan government is spent on health, education, and welfare.

To make such development projects effective you need security. That means more British soldiers, at least until sufficient Afghan forces have been adequately trained. That is the view of military commanders on the ground.

"The UK will be engaged in Afghanistan for a long time to come," Adam Thomson, Foreign and Commonwealth Office director for south Asia and Afghanistan, told the recent Rusi meeting. The burning question, which to the frustration of army chiefs is still not being properly answered in Whitehall, is engaged in what exactly?